Milk kept in open turns sour after some time due to-

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Lactic acid. Milk is about 87 percent water plus proteins, fats, minerals and lactose (milk sugar). Lactose is less in amount however for residual Streptococci bacteria which gobble it up and convert it into lactic acid. As lactic acid accumulates, the pH of milk increases to the point where Streptococci can no longer grow. The rising acidity literally sours the milk and causes the milk proteins to coagulate.

Milk kept in open turns sour after some time due to-

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